In the proposed work, intracellular microelectrodes will be used to simultaneously measure intracellular sodium ion concentration and membrane current under voltage-clamp conditions in mammalian heart muscle. Since tension will also be measured, it will be possible to characterize the dependence of tension on electrogenic sodium pump activity. Recent work has demonstrated the existence of this electrogenic pump in cardiac tissue. It is hoped that this work may clarify the mode(s) of action of cardiotonic steroids (such as digoxin) since circumstantial evidence suggests that such drugs act by blocking the Na pump. However, additional mechanisms (besides the sodium pump) also influence sodium transport (e.g. sodium-calcium exchange), and such mechanisms must be examined as well in order to determine the degree to which these mechanisms may control cardiac force generation.